Testing for the Presence of an Alkene

Key Concepts

• Alkenes

  • Contain a carbon-carbon double covalent bond.
  • Known as unsaturated hydrocarbons.
  • Double bond can break in an addition reaction allowing two new atoms to join.

• Alkanes

  • Saturated hydrocarbons with no double bonds.
  • No addition reactions due to lack of double bonds.

Characteristics

• Both alkanes and alkenes are colorless liquids (e.g., hexane and hexene).

Bromine Water Test

• Bromine Water

  • A solution of bromine dissolved in a solvent, appearing orange or yellowy brown.

• Reactions with Alkenes

  • Alkenes can undergo an addition reaction with bromine water due to unsaturation.
  • Double bond in alkenes (e.g., hexene) breaks, allowing bromine atoms to add across the bond.
  • Product formed: Dibromoalkane (colorless).
  • With excess alkene and small bromine water, solution turns from brown to colorless.

• Reactions with Alkanes

  • Alkanes are saturated; thus, no addition reaction occurs with bromine water.
  • Solution remains brown as no reaction takes place.

Summary

• Alkenes: Unsaturated, can react with bromine water to change color from brown to colorless.
• Alkanes: Saturated, do not react with bromine water; solution remains brown.